In various reproduction systems, including xerographic printing, the control and registration of the position of imageable surfaces such as photoreceptor belts, intermediate transfer belts, if any, and/or images on such imageable surfaces, and the control and registration of images transferred to and developed on a medium, such as for example, a sheet of paper, involve both initial and process control methods. Image on paper errors can show up as absolute position errors, magnification or shrinkage errors and image skew errors. Position errors may be caused by the paper alignment devices. Magnification and shrinkage errors may be caused due to shrinkage of paper in the fuser and errors in photoreceptors or Roster Output Scanner (ROS) polygon speeds. Skew errors are caused by ROS skew and errors in image alignment devices.
Current procedures for measuring or evaluating image on paper registration (IOP) on printing machines are extremely tedious, iterative and cumbersome. They involve printing internal test prints and manual measurement of different artifacts on each side of the paper with a flexible scale and inputting those values to the printer. This is done in a serial fashion, one factor at a time, resulting in a long setup time, setup errors and customer dissatisfaction.
One technique for measuring image to paper registration involves scanning the printed medium on a flatbed scanner. The image on the printed medium may then be compared to an ideal image and deviation from ideal may be determined. The measurements used, however, are limited to sheets smaller than the full capacity of a printer primarily due to the size of the scanner and the method of analysis. In addition, the precision of measurements tend to be limited by the accuracy of the scanner itself. In order to eliminate errors caused by the scanning process, a separate time consuming procedure is required for calibration of the scanner. Furthermore, some techniques tend to use only a few (3-4) points on the page for measurement, thus limiting the information that is gathered. As such, there is no opportunity to use averaging to help interpret spatial errors, nor is there enough information to understand the signature of the errors.
Furthermore, it is tedious and time consuming for a user to properly place the sheet on the platen. Image on paper registration measurement requires edge detection and, therefore, overscanning of the edges, which means that the user must create a gap between the edges of the sheet and the edges of the scan area. This is especially tedious when handling a large number of sample prints. An operator can easily fail to create a gap because the scan area is generally unmarked.
Accordingly, it would be desirable to provide a system for accurately measuring image on paper registration.